1. Field of the Invention
The present invention relates generally to RF (radio frequency) power transmission, modulation, and amplification.
2. Background Art
Today's RF power amplifiers are required to generate complex RF signals with stringent output power and linearity requirements. For example, in order to comply with the requirements of a WCDMA waveform, a power amplifier needs to support approximately 30-40 dB of instantaneous output power dynamic range at a given power output level. This is mainly due to the ACPR (Adjacent Channel Power Ratio) and the ACLR (Adjacent Channel Leakage Ratio) requirements of the WCDMA waveform, which require very deep nulls as the output power waveform crosses zero.
Generally, the ACLR and ACPR that a power amplifier can achieve are related to the linearity of the power amplifier over the output power range of the desired waveform. Modern RF waveforms (e.g., OFDM, CDMA, WCDMA, etc.) are characterized by their associated PAP (Peak-to-Average Power) ratios. As such, in order to generate such waveforms, the power amplifier needs to be able to operate in a largely linear manner over a wide output power range that encompasses the output power range of the desired waveforms.
Outphasing amplification or LINC (Linear Amplification with Nonlinear Components) provides an amplification technique with the desirable linearity to amplify RF waveforms with large PAP ratios. Outphasing works by separating a signal into equal and constant envelope constituents, linearly amplifying the constituents, and combining the amplified constituents to generate the desired output signal. To preserve linearity when combining the amplified constituents, existing outphasing techniques use an isolating and/or a combining element, which provides the needed isolation between the branches of the outphasing amplifier to reduce non-linear distortion.
In several respects, however, existing outphasing techniques are not suitable for implementation in modern portable devices. For example, the isolating and/or combining element that they use causes a degradation in output signal power (due to insertion loss and limited bandwidth) and, correspondingly, low power amplifier efficiency. Further, the typically large size of isolating/combining elements precludes having them in monolithic amplifier designs.
There is a need therefore for outphasing amplification systems and methods that eliminate the isolating/combining element used in existing outphasing techniques, while providing substantially linear amplification over a wide output power dynamic range to support modern RF waveforms.